... developers may be building record-breaking towers even though they know they are economically inefficient. There is, after all, a certain cachet to having a very tall building with your name on it. In 1998 Donald Trump, a magnate, presented a plan to build the world's tallest residential building in New York as the righting of a historical wrong, not a shrewd business move. "I've always thought that New York should have the tallest building in the world," he proclaimed. If such vanity projects can secure funding, the theory goes, financial markets must be out of control and will soon suffer a sharp correction. Mr Trump's tower opened just as the dotcom bubble was bursting.

Historical analysis suggests that developers are prone to bouts of irrationality. In a paper from 2010, Jason Barr of Rutgers University looked at 458 skyscrapers (those at least 100 metres tall) completed in Manhattan between 1895 and 2004. The number of skyscrapers built and their average height depended in part on the growth in population and employment in office jobs. But Mr Barr's calculations suggest that the height of towers was also shaped by those nearby, especially during economic booms. In the 1920s, Mr Barr estimates, New York builders added four to six more floors per project, just to stand out in the skyline.